Now scientists have added another coral-killing culprit to the mix: seaweed.

A new study of reefs around the Fiji Islands found that some seaweed causes coral bleaching and suppresses photosynthesis by emitting anti-coral chemicals.

Such seaweed could be helping to curb coral growth on reefs worldwide, the study authors say.

During the last several decades, scientists have observed numerous coral reefs being conquered by seaweeds. But experts haven't been sure if the algae contributed to coral decline or simply benefited from it.

"Now our research suggests that, once corals decline due to a combination of global and local stresses, some seaweeds use chemical warfare to suppress the recovery of remnant adult corals and new coral recruits," said study co-author Douglas Rasher, a doctoral candidate at the Georgia Institute of Technology in Atlanta.

It's unlikely that seaweed developed chemical weapons specifically to attack coral, since interactions between the two organisms have been historically infrequent on pristine reefs, Rasher said.

"We hypothesize that [seaweeds] may have evolved to produce these compounds on their surfaces in response to competition with microbes or attacks from [grazing animals], which they've been fighting for millions of years," he explained.

"It's only in ecologically recent times—perhaps over the last 30 or 40 years—that seaweeds have begun to have an impact on corals."

Two main forces are thought to be driving the seaweed boom on reefs. In some cases, agricultural and sewage runoff dumps large amounts of nutrients on near-shore reefs, stimulating algae growth.

Rasher added that Fiji's protected marine reserves could naturally supply new coral larvae to adjacent reef habitats, although the situation is unlikely to improve if seaweeds continue to suppress new coral recruits.

"Our results suggest that bans on the harvest of seaweed-eating fish may decrease algal abundance on the degraded reefs and thus make these reefs more prone to coral re-establishment," he said.